Electron beam tube with transverse wave modulation for the amplification of high-frequency signals



March 26. 1968 SEUNlK ET AL 3,375,459

ELECTRON BEAM TUBE WITH THANSVERSE WAVE MODULATION FOR THE AMPLIFICATIONOF HIGH-FREQUENCY SIGNALS Filed Aug. 6, 1965 2 Sheets-Sheet l INVENTQRSf/ozs/ Sean/ Werner Ve/Ph ATTYS.

March 26. 1968 H. SEUNIK ET AL- 3,375,459

ELECTRON BEAM TUBE WITH TRANSVEHSE WAVE MODULATION FOR THE AMPLIFICATIONOF HIGH-FREQUENCY SIGNALS Filed Aug. 6, 1965 2 Sheets-Sheet 2 TflglFig.7

- INVENTQRS flora) Sean/k War/7 @r Way/2 ATTYS.

United States Patent Claims ABSTRACT OF THE DISCLOSURE An electron beamtube with transversewave modulation, in which transverse waves in theelectron beam are amplified by a magnetostatic pump field, the pumpfield being a magnetic field alternating periodically in direction witha field strength course which presents a dip in the middle of each halfperiod of the magnetic field, and in which system the maxima of thefield course on both sides of the dip are separated on the respectivesides of the dip by at least a quarter of the period length, with theelectron beam being focused in the second or higher pass range.

The invention relates to an electron beam tube with transverse wavemodulation for the amplification of high frequency signals with an inputcoupling part, a drift space and an output coupling part, in .which therespective input and output coupling parts consist of a coupler fortransverse waves, with a magnetic system being provided which generatesa magnetostatic pump field over the length of the drift space withsuccessive magnetic means, the field strength of which pump fieldchanges spatially periodically in electron beam direction, while in thezones of the input coupling and output coupling parts there is present amagnetic field having substantially constant field strength.

Electron beam tubes of this type are already known, in which, for theamplification of the transverse waves (cyclotronic or synchronous waves)in the electron beam, there is utilized a magnetostatic pump fieldrunning paral-' lel to the electron beam axis, and which has a fieldcomponent spatially periodically changing in electron beam direction.For the generation of the spatial periodicity there is employed a seriesof successive cylindrical iron disks, which surround the electron pathaxially symmetrically.

Such -'an electron beam tube can be conceived as a parametric amplifierin which, in place of a high frequency pump, the spatially periodicmagnetic field produces an amplification of the transverse Waves. Theadvantage of such a tube lies in the fact that, as compared to theconventional traveling wave tubes, no delay line is required and nospecial attenuation. Moreover there is obtained at the output of thetube an electron beam with an essentially constant electron velocity, sothat all the electrons can be decelerated at the collector tosubstantially zero 'velocity, and thereby attain a high tubeefiicien'cy.'

' The invention has as its problem that of creating an electron beamtube with transverse wave. modulation which, while avoiding a highfrequency electric pump voltage, can operate with an electron beam ofvery high perveance.,For the solution of this problem it is proposed,according to the invention, that in an electron beam tube of the typepreviously mentioned, the magnetostatic pump field be a magnetic field,periodically alternating in direction, with a field strength coursewhich always presents a dip in the middle'of a half period of themagnetic field, with the maxima of such field course at both sides ofthe ice dip being spaced apart at least a quarter of the period length.The period length of the magnetic field is so selected with reference tothe electron beam velocity that the border of the electron beam presentsat least two bulges during a halfperiod of the magnetic field.

The amplification system of an electron beam tube according to theinvention is freely selectibly wide-banded, because for each fed-infrequency the spatially periodic field always appears as pump withdouble pump frequency. Further, since the beam location in the tube isnon-critical, the problem of conducting electron beam with a certaindiameter as closely as possible past any electrodes, does not exist.

The invention will hereafter be explained in detail, with the aid of thedrawings, in which:

FIG. 1 is a schematic figure illustrating the construction of a tubeaccording to the invention with a portion of the enclosing envelopebroken away to show tube details;

FIG. 2 is a graph representing the magnetic field course;

FIG. 3 is a graph illustrating the field strength course of themagnetostatic pump field;

FIGS. 3, 4 and 5 are related figures respectively directed to the fieldstrength course of the magnetostatic pump field, the nature of the beamborder oscillations of a Brillouin beam, and the magnetic means for thegeneration of the magnetic field B(z);

FIG. 6 is a schematic figure illustrating a portion of a pratical magnetarrangement, of the type illustrated in FIG. 5;

FIG. 7 is a graph illustrating the magnetic field parameter a, themovement of the center of gravity of the beam, and the representation ofthe nature of the border of such a beam; and

FIG. 8 is a graph illustrating the potential travel variation of theelectrons of the electro beam in relation to their disposition withrespect to the beam axis and the drift tube.

Referring to FIG. 1, reference numeral 1 designates an electron beamgenerating system, the generated electron beam of which is to permeatesuccessively along the discharge path 2, an input coupling part 3, adrift tube 4 and an output coupling (decoupling) part 5, prior to theelectron beam striking the collector 6, both the input coupling part 3and the output coupling part 5 comprising couplers for transverse waves.In the particular embodiment illustrated, the couplers are constructedin the manner of a socalled Cuccia coupler and consist in each case of arectangular hollow conductor in which an electromagnetic oscillation isexcited by means of an electrical alternating field transverse to theelectron beam axis. Such rectangu:

lar hollow conductor is provided with two tuning slides 7, arrangedsymmetrically at both sides of the hollow conductor, making it possibleto mechanically tune the coupler to the particular signal frequency.Such a tunning can be a omitted if, in an advantageous manner, insteadof a Cuccia coupler for the respective coupling parts there is utilizedone of the known wide-band couplers for transverse waves, for example inthe form of a delay line, which conducts a transverse electricalalternating field.

As is apparent from FIG. 2, in the respective zones of the couplingparts there is present a magnetic field Sor 12 with substantiallyconstant field strength, which, with the use of the Cuccia couplerillustrated, is of such a magnitude that the cyclotron frequency isequal to the signal frequency. The magnetic field 8 with a briefincrease in its field strength (magnetic transition field 9), mergesinto the magnetic pump field 10, which extends over the length of thedrift tube 4. The pump field 10, according to the invention, is amagnetic field periodically alternating in direction with a fieldstrength course which has a dip in the middle of a half period of themagnetic field 9. The detailed conditions of this clip as well as themanner of operation of the pump field are hereinafter further explained.

It should be noted here, however, that through the pump field 9 at theinput coupling part 3 the electron beam, modulated with a transversewave, for example cyclotron wave, is focussed as a whole and also thetransverse wave is amplified in the beam. After leaving the drift tube,the electron beam passes through another transition field 11, from whichthe pump field 10 merges into the magnetic field 12 of constant fieldstrength. There is associated with the transition field 11 the advantagethat an amplified cyclotron wave receives a clear wave form in theelectron beam.

The magnetostatic pump field of an electron beam tube according to theinvention is represented, with additional details, in FIGS. 3 to 5. FIG.3 illustrates the course of the magnetic field strength B(z) withrespect to the coordinate z (coordinate in longitudinal direction'of thebeam path). Therebelow, in FIG. 4, there is depicted the nature of theborder oscillations of a Brillouin beam 13 conducted in the magneticfield B(z), while FIG. schematically represents the magnetic means forthe genera tion of the magnetic field B('z) of FIG. 3.

It is well known that the movement of an electron beam in the manner ofa so-called Brillouin beam in a magnetic field of sine form is describedby a Mathieu equation. The solution of this equation divides intoso-called stability and instability ranges. In the instability ranges adeflection of the electrons of the electron beam from the axis 'isexponentially increased, so that transverse waves,

for example cyclotron waves, in passage through such a magnetic fieldare amplified in the instability range. It is necessary, however, thatthe electron beam in its totality be preserved. The beam itself, whichis characterized by the motion of its border electrons, thus should bemaintained. This is possible, despite a so-called positional instabilityof the individual electrons, in an instability range ofthe solution of aMathieu equation if the space charge field in the interior of the beamis included. From the Mathieu equations it results that for aconsideration of the stability of the whole beam the same differentialequation is obtained as for a space-charge-free electron beam with anadditional space charge term. From this it follows that ranges must bepresent in which the electron beam itself is stable, while shifting ofthe beam electrons from the beam axis, as a consequence, creates aninstability of position. A more precise mathematical investigation hasnow shown that in use of a magnetic field with a sine form course of themagnetic field strength the instability range involved for anamplification of transverse waves isassociated with a theoretical beamfocussing, which is practically unusable, as the minimal beam radiustakes on values close to zero.

However, considerable space charges can be focussed with a relativelysmall undularity of the border of the electron beam if a magnetic fieldof the type illustrated in FIG. 3 is employed. While this magnetic fieldhas a field strength alternating periodically in direction, it does nothave a sine form course of the magnetic field strength. On the contrary,the field strength course presents in the middle of each half period ofthe magnetic field, in which L represents the period length, a dip inwhich the maxima B of the magnetic field strength which bound the dipare separated by at least a quarter of the period length L. The electronbeam 13 according to FIG. 4 is focussed in this magneticfield in theso-called second pass range, that is, the range in which the number ofborder oscillations of the electron beam during a half period of themagnetic field amounts to 2. It should be noted that a focussing of theelectron beam in a still higher pass range also is possible. (Theordinal number of the pass range is defined as the number of whole beamborder oscillations of the electron beam during a half period of themagnetic field). With increasing ordinal number of the pass range themagnetic field maxima on both sides of the dip should be disposed 4further and further apart in the course of the original field strength.

For the generation of the magnetic field course. described there areexpediently used magnetic means accord ing to FIG. 5, which consist ofperiodically arranged magnetic poles 14 and 15, which alternate inpolarity along the electron path. Centered beween the respectivemagnetic poles 14 and 15 is, in each case, a tubular shield 16 of softmagnetic material, which causes the dip in the field strength courseB(z).

FIG. 6 illustrates a section from a practical magnetic arrangement ofthe type disclosed in FIG. 5, and comprises permanent magnets (notillustrated), which "are arranged symmetrically about the electron beamaxis z. Opposed, like poles of such permanent magnets, should bealternately connected with one another by pole pieces 17 and 18extending transversely to the electron beam axis. The thickness of thepole pieces 17 and 18, as viewed in electron beam direction, is reducedin the proximity of the beam axis 2 with respect to the outwardlydisposed parts of the pole pieces. Centered between the two pole pieces17 and 18 is a shielding cylinder 19, disposed atthe height of therectangular ofi'sets 20 and 21 on the pole shoes 17 and 18. With thedimensional proportions illustrated of the pole shoes 17 and 18, as wellas of the shielding cylinder 19, there results the course, representedin broken lines in FIG. 6, of the magnetic field strength over a halfperiod of the pump field in an electron beam tube according to theinvention.

The results of a calculation for a magnetic pump field whose magneticfield strength course is described by the function are represented inthe diagram of FIG. 7, and, in particu lar, for a certain so-calledmagnetic field parameter of the value 3.8. The parameter is determinedthere from the relation I f(z)=sin 2 40.5 sin beam (curve 22), in whichthere is seen only the projection upon a plane which turns about thebeam axis with.

the rotation of the electrons. Curve 22 is devised from the'factf thatin the calculation the individual electrons are consideredthat is, theMathieu equation is solved without the inclusion of a memberrepresenting the space charge (thread beam free of, space charge).Correspon ing to this curve, a transverse wave is amplified in theelectron beam. However, there is obtained ijor the whole electron beam,with account taken of the space charge, a beam border corresponding tocurve 23 of FIG. 7, the electron beam is accordingly conducted inbundled form in the magnetic field 8(2). The calculation further showsthat considerable space charge can be focussed, and simultaneously anamplification of a transverse wave 'is achievable in the electron beamof about 10 to 20 db per magnetic field period.

An electron beam tube according to the invention e'xhibits like thealready known electron beam tubes with transverse wave modulation, thephenomenon that the electron beam velocity diminishes in the directionof the discharge path when the electrons, in consequence of theamplification efiect, leave the beam and cross at an angle which becomesgreater over the length of the drift space. Theelectrons then havemaximal velocity on the path inclined to the beamaxis, so that theirvelocity, in the directionof the discharge path, is slower. This has theconsequence of creating saturation manifestations or non-' linearities.It is possible, however, to eliminate this drawback if the variation intravel potential of the electron beam is utilized in order toreaccelerate the electrons in the drift tube. By travel potential thereis meant the fact that the electrons have on the axis of the electronbeam a potential which is lower than at the beam border, while, in turn,the potential at the beam border is less than the potential of the drifttube surrounding the drift space. For the better understanding of thisthe travel potential mentioned is represented in FIG. 8, in which theaxis of the electron beam is designated by z, the beam border by r andthe drift tube by R. For a beam of, for example, 6 a. and 10 kv., whichcorresponds to a perveance P=6.l' there results according to the formulaA U: UR- U,= 1.5220 1 U-1/2(1+4.64 lg g) an interior travel potential inthe electron beam of AU=910 v.

(in the formula U represents the voltage on the drift tube, U thepotential on the beam axis, R the radius of the drift tube and r thebeam radius). For a ratio of R/r=5 there result for the outer apotential of 3160 v. This outer travel potential between the beam borderand the drift tube results in the electron beam being cormoves far awayfrom its axis. It is now possible to so select the perveance, the beamdiameter and the drift tube diameter that in the event of a deflectionof the electrons from the beam axis the effective velocity of theelectron beam remains constant and, under these conditions, the tubeoperates linearly. This is always the case when the desired travelpotential, in deflection of the electron beam from the axis of the drifttube, satisfies the formula 2 -i dtl a in which L designates the periodlength of the magnetic field and p the radius vector in the drift tube.

Changes may be within the scope and spirit of the appended claims whichdefine what is believed to be new and desired to have protected byLetters Patent.

We claim:

1. In an electron beam tube with transverse wave modulation for theamplification of high frequency signals, the combination of an inputcoupling part, a drift space and a decoupling part, the input couplingand decoupling parts comprising a coupler for transverse waves, amagnetic arrangement which generates over the length of the drift space,with successive magnetic means, a magnetostatic pump field, the fieldstrength of which changes spatially periodically in electron beamdirection, while in the vicinity of the input coupling and decouplingparts a magnetic field with essentially constant field strength ispresent, said magnetostatic pump field being a magnetic fieldalternating periodically in direction, with a field strength coursewhich always shows a dip in the middle of a half period of the magneticfield, the maxima of the field course on both sides of the dip beingspaced apart at least a quarter of the period length, and the periodlength of the magnetic field being such in relation to the electron beamvelocity that the border of the electron beam presents at least twobulges during :a half period of the magnetic field.

Z. An electron beam tube according to claim 1, wherein the drift spaceof the tube is surrounded by a metallic, non-magnetic drift tube.

3. An electron beam tube according to claim 2, wherein the drift tube ischarged with such a direct voltage U and its radius R is such that therelation eet 1 is fulfilled, in which U is the potential on the electronbeam axis and L the period length of the magnetostatic pump field.

4. An electron beam tube according to claim 1, wherein the magneticarrangement for the generation of the pump field consists of permanentmagnets which are symmetrically so arranged about the electron beamaxis, and are so connected with .pole pieces disposed transversely tothe electron beam axis, that successively following pole pieces arealways oppositely polarized, there being provided, in each case, in themiddle between successive pole pieces a soft magnetic hollow shieldingcylinder surrounding the electron axis.

5. An electron beam tube according to claim 4, wherein the pole piecesas viewed in electron beam direction, are provided with rectangularoffsets, reducing the size of the pieces in the electron beam axis withrespect to the outwardly disposed parts of the pole pieces, and theshielding cylinder is arranged at the level of the correspondingrectangular offsets on the pole pieces.

6. An electron beam tube according to claim 1, wherein the magneticfield in the vicinity of the input coupling part and the decoupling partmerges in each case into the magnetostatic pump field with a briefelevation of its field strength.

7. An electron beam tube according to claim 6, wherein the inputcoupling part and decoupling part each consist of a tunable rectangularhollow conductor, in which an electromagnetic oscillation is excitedwith an electric alternating field transverse to the electron beam axis.

8. An electron beam tube according to claim 7, wherein the rectangularhollow conductor is provided with two tuning slides arrangedsymmetrically to the electron beam axis.

9. An electron beam tube according to claim 6, wherein the inputcoupling part and the decoupling part each consist of at least one delayline on which an electromagnetic wave progresses with electric fieldcomponents directed transversely to the electron beam axis.

References Cited UNITED STATES PATENTS 3,072,817 1/1963 Gordon 330-4]JOHN KOMINSKI, Acting Primary Examiner. DARWIN R. HOSTEITER, Examiner,

